Atmos. Chem. Phys., 16, 13601–13618, 2016 www.atmos-chem-phys.net/16/13601/2016/ doi:10.5194/acp-16-13601-2016 © Author(s) 2016. CC Attribution 3.0 License. Unexpectedly acidic nanoparticles formed in dimethylamine–ammonia–sulfuric-acid nucleation experiments at CLOUD Michael J. Lawler1,a,b, Paul M. Winkler2, Jaeseok Kim3,4, Lars Ahlm5, Jasmin Tröstl6, Arnaud P. Praplan7,8, Siegfried Schobesberger7,9, Andreas Kürten10, Jasper Kirkby10,11, Federico Bianchi7, Jonathan Duplissy7, Armin Hansel12, Tuija Jokinen7, Helmi Keskinen3,7, Katrianne Lehtipalo7,6, Markus Leiminger12, Tuukka Petäjä7, Matti Rissanen7, Linda Rondo10, Mario Simon10, Mikko Sipilä7, Christina Williamson10,13,14, Daniela Wimmer7,10, Ilona Riipinen5, Annele Virtanen2, and James N. Smith1,b 1Department of Chemistry, University of California, Irvine, Irvine, CA, 92697, USA 2Faculty of Physics, University of Vienna, 1090 Vienna, Austria 3Department of Applied Physics, University of Eastern Finland, Kuopio, Finland 4Arctic Research Center, Korea Polar Research Institute, Yeonsu-gu, Incheon 21990, Republic of Korea 5Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden 6Paul Scherrer Institute, Villigen, Switzerland 7Department of Physics, University of Helsinki, 00014 Helsinki, Finland 8Finnish Meteorological Institute, 00101 Helsinki, Finland 9Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA 10Institute for Atmospheric and Environmental Sciences, Goethe University of Frankfurt, 60438 Frankfurt am Main, Germany 11European Organization for Nuclear Research (CERN), Geneva, Switzerland 12Institute for Ion and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria 13Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA 14Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA avisitor at: National Center for Atmospheric Research, Atmospheric Chemistry Observations and Modeling Lab, Boulder, CO, 80301, USA bformerly at: University of Eastern Finland, Department of Applied Physics, Kuopio, Finland Correspondence to: Michael J. Lawler ([email protected]) Received: 27 April 2016 – Published in Atmos. Chem. Phys. Discuss.: 24 June 2016 Revised: 16 September 2016 – Accepted: 29 September 2016 – Published: 3 November 2016 Abstract. New particle formation driven by acid–base chem- for the collected 10–30 nm VMD particles. This behavior istry was initiated in the CLOUD chamber at CERN by intro- is not consistent with present nanoparticle physicochemical ducing atmospherically relevant levels of gas-phase sulfuric models, which predict a higher dimethylaminium fraction acid and dimethylamine (DMA). Ammonia was also present when NH3 and DMA are present at similar gas-phase con- in the chamber as a gas-phase contaminant from earlier ex- centrations. Despite the presence in the gas phase of at least periments. The composition of particles with volume median 100 times higher base concentrations than sulfuric acid, the diameters (VMDs) as small as 10 nm was measured by the recently formed particles always had measured base : acid ra- Thermal Desorption Chemical Ionization Mass Spectrome- tios lower than 1 : 1. The lowest base fractions were found in ter (TDCIMS). Particulate ammonium-to-dimethylaminium particles below 15 nm VMD, with a strong size-dependent ratios were higher than the gas-phase ammonia-to-DMA ra- composition gradient. The reasons for the very acidic com- tios, suggesting preferential uptake of ammonia over DMA position remain uncertain, but a plausible explanation is that Published by Copernicus Publications on behalf of the European Geosciences Union. 13602 M. J. Lawler et al.: Acidic nanoparticles at CLOUD the particles did not reach thermodynamic equilibrium with particles grow, and water vapor has been shown to be an im- respect to the bases due to rapid heterogeneous conversion of portant contributor to the initial steps of new particle forma- SO2 to sulfate. These results indicate that sulfuric acid does tion (e.g., Zollner et al., 2012, and references therein). While not require stabilization by ammonium or dimethylaminium ammonia is present at much higher concentrations than DMA as acid–base pairs in particles as small as 10 nm. in the remote atmosphere, amines such as DMA can effec- tively compete with ammonia in the growth of nanoparti- cles of ∼ 10 nm diameter (Smith et al., 2010; Barsanti et al., 2009). Measurements made at the CLOUD chamber in 1 Introduction CERN of molecular clusters in the sulfuric acid–base sys- tem show a nucleation process involving stepwise additions Atmospheric new particle formation (NPF) refers to the for- of H2SO4 and base, typically in a 1 : 1 ratio (Kirkby et al., mation and subsequent growth of condensed-phase particles 2011; Schobesberger et al., 2013, 2015; Almeida et al., 2013; from gas-phase precursors. The mechanisms and chemical Kürten et al., 2014; Bianchi et al., 2014). At this early stage species responsible for NPF have been the subject of many in the particle formation process, when there are fewer than recent studies, with the motivation of understanding the im- about 20 molecules in the clusters, ionic acid–base interac- pacts of these processes on air quality and climate. NPF is an tions clearly drive the growth. For these experiments, sulfuric important source of cloud condensation nuclei (CCN) and acid was present at much lower concentrations than were the therefore may significantly affect the global radiative en- bases, so it was the chemical species that limited the growth. ergy balance (Wang and Penner, 2009; Kazil et al., 2010), Here we present chemical composition measurements of but the mechanisms and chemical species driving NPF are recently formed nanoparticles in the size range of 10–30 nm still poorly understood (e.g., Riipinen et al., 2012). Sulfuric in diameter (volume median diameter for collected parti- acid is widely accepted as one of the most important species cles), from experiments conducted in the CLOUD chamber for particle formation in the atmosphere (e.g., Kirkby et al., at CERN. These are the first direct compositional measure- 2011; Kuang et al., 2008; Kulmala et al., 2007; Weber et ments of newly formed particles in this chemical system. al., 1997). However, particle growth rate and composition These observations place constraints on the nature of parti- measurements have indicated that species other than sulfu- cle growth after nucleation in the DMA–NH3–H2SO4–water ric acid dominate growth past the smallest particle sizes in system, with implications for new particle formation in the many environments (O’Dowd et al., 2002; Weber et al., 1997; atmosphere. Smith et al., 2008; Kuang et al., 2010; Bzdek at al., 2012). Three fundamental types of growth pathways for nanopar- ticles have been identified in the literature: reversible con- 2 Laboratory conditions densational growth by low-volatility species, reactive up- take, and acid–base interactions (Riipinen et al., 2012, and The experiments were undertaken at the seventh Cosmics references therein). Recent evidence has supported an im- Leaving Outdoor Droplets (CLOUD7) campaign at CERN portant role for highly oxidized, very low-volatility organic during October 2012. The CLOUD chamber is a 26.1 m3 molecules in the growth of atmospheric nanoparticles (Ehn cylindrical chamber made from electropolished stainless et al., 2014; Zhao et al., 2013; Riccobono et al., 2014). Reac- steel. Efforts were taken in its construction to make it the tive uptake involves the multiphase transformation of a gas- cleanest possible chamber used for studying atmospheric phase species into a new condensed-phase species that is less particle nucleation (Kirkby et al., 2011; Schnitzhofer et al., volatile than its precursor (e.g., Wang et al., 2010). Growth 2014; Duplissy et al., 2016). It can be precisely temperature due to acid–base chemistry results from strong ionic inter- controlled, and UV light at 250–400 nm can be provided to actions, which could be reversed depending on the chemical the chamber with minimal heating via an array of fiber-optic conditions within the cluster or particle (Bzdek et al., 2010; bundles (Kupc et al., 2011). It is possible to eliminate ions Barsanti et al., 2009). from the chamber using a 20 kV m−1 clearing field and con- In the sulfuric acid–dimethylamine–ammonia–water sys- versely to increase the number of chamber ions by directing a tem, the most likely growth pathways involve electrostatic 3.5 GeV pion beam into the chamber. The latter state results interactions arising from proton transfer from sulfuric acid in ion concentrations that are up to an order of magnitude to the base species, direct condensational growth by sulfuric greater than what is present naturally from cosmic ray bom- acid due to its very low saturation vapor pressure, and coag- bardment at the Earth’s surface (Franchin et al., 2015). ulation of particles and/or clusters. Dimethylamine (DMA) Clean air and reagents were added to the chamber on a and ammonia on their own are quite volatile gases, and sig- continuous basis, yielding a roughly 4 h overturning time for nificant growth by water vapor condensation alone is unfa- the chamber. The chamber conditions were set to simulate vorable due to the Kelvin effect until the particles are larger atmospheric conditions relevant to the lower troposphere. than about 50 nm in diameter. Nonetheless, all of these com- The main constituent gases